This invention relates to mercury-free metal halide arc discharge lamps, and, more particularly to the use of rare earth iodides as a replacement for scandium iodide in low-wattage, mercury-free arc discharge lamps in order to provide improved photometric and electrical characteristics.
The metal halide lamp is an improvement to the mercury lamp. In addition to mercury and noble gas, the lamp also contains salts of elements that emit desired radiation. Designers specify metal halide lamps in high power applications such as streetlights and high bay illumination. But more and more metal halides are being designed for lower power applications as lamp and system technology improves.
Sodium/scandium chemistry is well known as an emitter of visible light. Indeed, it is the preferred chemistry for most of the metal halide type of lamps manufactured for general illumination in the United States today. Its primary asset is initial efficacy. With sodium/scandium chemistry, the larger lamp types easily produce over 100 lumens per watt (LPW). Another advantage is the correlated color temperature (CCT). A range of CCT from 3000 Kelvin to 5000 Kelvin is readily obtained by varying the sodium to scandium ratio.
The disadvantage of this system, however, is a poor color rendering index (CRI). Typically, sodium/scandium lamps produce light with a CRI of only 65 Ra. Although this value allows their use for street lighting in the United States, other countries, especially in Europe, prefer significantly higher CRI values.
A more serious disadvantage of the sodium/scandium chemistry arises from the reaction of the scandium with the silica in the envelope wall. This reaction progresses more rapidly above approximately 800xc2x0 C. Unfortunately, for standard mercury-metal halide lamps, this is below the temperature at which significant vaporization of the chemicals occurs and the temperature at which the lamp would begin to produce extraordinary performance characteristics. Consequently, lamp engineers sacrifice some performance in order to keep the temperature low and prolong life.
One of the reaction products is scandium silicate; the other is excess free-iodine. Excess free iodine reduces the efficacy of the sodium/scandium lamps; it increases the required voltage, and affects the CCT and the CRI. In sodium/scandium lamps containing mercury, the scandium-silica reaction with the ensuing excess free iodine contributes to early lumen maintenance failure, a major problem with commercially available sodium/scandium metal halides. In mercury-free lamps, the free iodine not only reduces the efficacy of the lamp but constricts the arc, thus raising the lamp voltage and increasing the xe2x80x9ccrest factorxe2x80x9d, or re-ignition voltage.
In addition to the desire to avoid the problems of sodium/scandium lamp chemistry and to improve inherent lamp characteristics such as CCT, and CRI, and to improve efficacies, there are concomitant desires to reduce manufacturing costs, and to eliminate toxic materials from the metal halide discharge lamps. In this regard there is much effort to eliminate mercury from the lamp chemistry.
The removal of mercury from arc lamps is not an easy goal since mercury is an ideal arc medium. It is a liquid with a low vapor pressure at room temperature. Thus, it is easy to strike and sustain an arc. At operating temperatures, however, the mercury is completely vaporized and the pressure becomes quite high. The voltage across the lamp increases to the point where cost effective, efficient power supplies can be designed to drive the lamp.
Several metal halide lamp technologies do not use mercury in the arc tube. However, previous attempts at mercury-free lighting systems have been plagued with poor efficiency, poor color, poor life, and have been restricted to horizontal operating conditions. One such technology uses an arc discharge tube without electrodes and is driven by microwave or radio frequency energy. This technology requires a microwave source and a means to couple the energy to the arc tube. Generally, the arc tube must be in close proximity to the wave-guides used as the means of coupling.
Another mercury-free metal halide technology uses a pulsed ballast source to operate the arc discharge tube. This technology requires an arc tube with electrodes at opposing ends and a power source that generates narrow pulses across the anode and cathode. Light is emitted during the pulse phase; so the power supply must generate repetitive pulse to achieve the perception of constant light. One patent describing this technology is U.S. Pat. No. 4,874,988 issued Oct. 17, 1989 and assigned to the assignee of the instant invention.
Another mercury-free technology is taught in U.S. Pat. No. 4,757,236 which teaches a mercury-free sodium lamp using an alumina arc tube and xenon buffer gas to achieve high luminous efficiency. The patent does not teach or suggest any arc tube materials other than alumina. This patent also teaches using high xenon pressure as a buffer gas to achieve reduced wall reactions. It describes the need for additional starting requirements for such a design. This patent also discloses a second metal halide in combination with sodium iodide.
The present invention addresses the deficiencies discussed hereinabove. Apparent to those skilled in the art is that many of these parameters are dependent on one another or related to yet other variables. In some cases the parameters may actually be inversely related. Therefore, it has been very difficult to create a system for all characteristics. In these cases, improving even one parameter while not degrading any others is a significant step forward in the art.
It is, therefore, an object of the invention to obviate the disadvantages of the prior art.
Yet another object is to provide a discharge arc lamp with improved color rendition index values.
Another object of the present invention is to provide an arc medium composition that is less reactive with the arc tube and the electrodes.
Still another object of the current invention is to provide a discharge arc lamp with a color temperature and color rendition index that remain constant over the life of the lamp.
Yet another object of the current invention is to provide a discharge arc lamp whose efficacy, measured in lumens per watt, degrades little over the lifetime of the lamp.
Additionally, an object of the instant invention is to provide a discharge arc lamp that is more environmentally acceptable by removing mercury from the arc medium.
Furthermore, an object of the invention is to provide a discharge lamp that is unlikely to burst by maintaining a low pressure within the arc discharge tube.
Yet another object of the present invention is to provide a lamp manufacturing process and arc chemistry that are both inexpensive and reliable.
The present invention is an outcome of the desire to improve upon the known deficiencies of the industry-standard sodium/scandium discharge arc lamp technology. Specifically, it is known that this arc composition suffers from two rather specific deficiencies: a low color rendition index (CRI) value of approximately Ra65; and a composition that is inherently reactive with the glass arc tube. In the former case, the low CRI precludes selling the lamps for certain applications in certain localities, such as in Europe where CRI values must be exceed 80 before commercial success can be achieved. In the latter case, photometric and efficiency characteristics change dramatically over the lifetime of the lamp.
A second desire is to eliminate hazardous and environmentally unacceptable components from the lamp discharge medium. As evidenced from the patents cited supra, the industry has long had the goal of removing mercury from the arc medium. This desire has proved problematic since mercury plays an important part in lamp operation and a beneficial role in lamp photometric quality and overall efficiency. Indeed, studies have shown that the efficiency characteristics of the sodium/scandium are made significantly worse when mercury is removed from the arc medium since operating voltages fall off significantly. In the absence of mercury, lamps become difficult to start and require alternative technologies to compensate for the loss in the voltage gradient. Furthermore, at reasonable amperage, the power into the lamp is insufficient to raise the envelope temperature high enough to vaporize the salts. One method to increase the voltage and the power is by increasing the pressure of a noble gas in the arc medium. However, this makes the lamp difficult, if not impossible to start.
In the present invention, the decrease in voltage gradient caused by the removal of mercury is partially compensated by modifying the aspect ratio of the lamp design (see co-pending application Ser. No. 09/413,923, filed concurrently herewith). The aspect ratio is defined as the distance between the two electrodes in the arc tube divided by the diameter of the arc tube. For acceptable voltage gradients, the aspect ratio should be about 5:1 but values up to 15:1 still provide adequate results. When this lamp design is utilized with the standard sodium/scandium iodide discharge medium, the lamp performs acceptably, except that the CRI values are still unacceptably low. It has now unexpectedly been found that the replacement of scandium iodide with one or more rare earth iodides can still yield acceptable efficacies and can provide for increased CRI values and allow for additional benefits such as stable photometric values during the useful life of the lamp.